Interactions between Deliberation and Delay-Discounting in Rats

Article excerpt

Abstract When faced with decisions, rats sometimes pause and look back and forth between possible alternatives, a phenomenon termed vicarious trial and error (VTE). When it was first observed in the 1930s, VTE was theorized to be a mechanism for exploration. Later theories suggested that VTE aided the resolution of sensory or neuroeconomic conflict. In contrast, recent neurophysiological data suggest that VTE reflects a dynamic search and evaluation process. These theories make unique predictions about the timing of VTE on behavioral tasks.We tested these theories of VTE on a T-maze with return rails, where rats were given a choice between a smaller reward available after one delay or a larger reward available after an adjustable delay. Rats showed three clear phases of behavior on this task: investigation, characterized by discovery of task parameters; titration, characterized by iterative adjustment of the delay to a preferred interval; and exploitation, characterized by alternation to hold the delay at the preferred interval.We found that VTE events occurred during adjustment laps more often than during alternation laps. Results were incompatible with theories of VTE as an exploratory behavior, as reflecting sensory conflict, or as a simple neuroeconomic valuation process. Instead, our results were most consistent with VTE as reflecting a search process during deliberative decision making. This pattern of VTE that we observed is reminiscent of current navigational theories proposing a transition from a deliberative to a habitual decision-making mechanism.

Early behavioral theories of VTE suggested that VTE occurred during investigation of alternatives (Tolman, 1948), while later theories suggested that VTE occurred as a result of conditioned orienting (Bower, 1959; Spence, 1960). Recently, Krajbich, Armel, and Rangel (2010) observed saccade- fixate-saccade (SFS) sequences in humans making decisions between snack foods; these human sequences share similar properties to VTE in rats. Subjects showed more SFS when the value between the choices was equal, suggesting an explanation for VTE based on an underlying neuroeconomic valuation process (Glimcher, Camerer, & Poldrack, 2008; Krajbich et al., 2010).

Here, we examine the behavioral timing of vicarious trial and error on a spatial delay-discounting task that dissociates exploratory, conditioned orienting, and value equalization explanations. …